Allow to display Axis.Time labels like in Axis.Category.

Close #1130.

4 different configurations of "center" and "between" layouts

http://jsfiddle.net/kr5eapzt/41/

![screen shot 2014-12-18 at 3 55 44 pm](https://cloud.githubusercontent.com/assets/8551049/5498101/7cfc916e-86ce-11e4-865d-3768b6992c0c.png)

It seems like it would make more sense to have a `D3.Selection` containing all instances of a given container or baseline, rather than an array?